In an area like the Long Island Sound, how often should a chartkit be
updated?
Are 5 or 8 year old charts useless? I'm getting a very wide variant of
opinions
on this one.

Its a function of how frequently aids to nav are changed. Follow the notice to
mariners and see how often changes hit your area. I had a chart for the Great
South Bay that was 20 years old and it was fine, except all the buoys had their
numbers changed, even though they were still in the same place. It made
picking my way through a little tough but not impossible.